The present invention relates generally to High Intensity Discharge (HID) lamps and electronic ballasts for powering such lamps. More particularly, the present invention relates to various embodiments of an inrush protection circuit for use in an electronic ballast for powering an HID lamp.
Generally stated, an HID electronic ballast includes a tank circuit which stores energy so that the ballast can continue to power the load (i.e., an HID lamp) for a certain period of time even if the input power source to the ballast is disconnected for some reason, such as a black out. This function is required because an HID lamp will not restart immediately after extinguishing until it is cooled down. To minimize operational inconvenience, the ballast should not extinguish the lamp during very short-term black outs, in the order of tens of milliseconds.
A second function of the tank circuit is to smooth out the AC input ripple so that an output power regulation circuit which connects to the tank circuit can regulate the output to the HID lamp without having any adverse effect from the input ripple. The higher the capacitance in the tank circuit, the lower the AC ripple that will appear on the rectified bus voltage, but the higher the spike current to charge the tank circuit capacitor.
In designs where a high spike current is presented to charge the tank circuit capacitor when the power is initially applied, the spike current needs to be controlled. This spike current is also known in the art as an inrush current. Generally, the inrush current needs to be less than a certain value to avoid problems such as, for example, the welding of switches or terminals which are incorporated within the ballast due to the high peak current, or to prevent the circuit breaker which is in series with the input power source and the lighting fixtures from becoming active.
A common method for preventing excessive inrush current is to add resistive impedance into the circuit of the ballast. The resistive impedance limits the peak of the inrush current at initial power up. At the same time, a switch is added in parallel with the resistive impedance. The switch will be turned on to short the resistive impedance when the tank circuit has been charged up enough not to have too high peak current even with the shorted resistive impedance. In this configuration, the inrush current flows through the resistive impedance, while in normal operation the input current flows through the switch. This circuit arrangement which limits the inrush current and bypasses the inrush current limiting element for normal operation is called an inrush protection circuit.
FIG. 1 shows a first example of an inrush protection circuit 11 in a ballast 1 as conventionally known in the art having a boost chopper as a PFC (Power Factor Correction) circuit. This inrush protection circuit 11 is relatively simple. The switching element Q1 is passively controlled. DB1 is the diode bridge and it rectifies the AC input power source. R1 is a resistor which is provided along the low potential side (i.e., ground, or alternatively stated the negative terminal of the circuit) of the rectified input voltage. Q1 is a MOSFET switch and is coupled in parallel with resistor R1.
R2 and R3 are resistors which divide the rectified input voltage and provide the voltage at the gate of switch Q1. C2 is a capacitor coupled across the gate of switch Q1. L1, D1, and Q2 are an inductor, diode and MOSFET, respectively, and they collectively form a boost chopper PFC circuit. C0 is a film capacitor which can carry the high frequency current for the PFC circuit. C1 is an electrolytic capacitor and it is further part of the tank circuit of the ballast. Capacitor C1 has enough capacitance to deliver power to the load, which includes the output power regulation circuit, for a certain period of time when the input power source is disconnected in normal operation. The capacitance is also large enough to filter out the AC ripple voltage after rectification.
When the input power source is connected to the ballast, the gate voltage of switch Q1 is gradually charged up by the rectified input voltage, depending on the divider ratio between resistors R2 and R3, and further depending on the time constant between resistor R2 and capacitor C2. When switch Q1 is off, the inrush current delivered from the input power source to charge up capacitor C1, the tank circuit, flows through a circuit loop consisting of diode-bridge DB1, inductor L1, diode D1, capacitor C1, and R1 as the resistive impedance. The peak of the inrush current is mainly controlled by resistor R1. At the same time, capacitor C1 is also being charged up by the boost chopper. When the gate voltage of the switch Q1 exceeds the threshold voltage of the switch Q1, switch Q1 turns on. After switch Q1 turns on, the input current flows through switch Q1 instead of resistor R1. If capacitor C1 is charged up more than the peak of the input voltage when switch Q1 turns on, no high peak charging current flows through C1 and switch Q1.
There are three design parameters for this inrush protection circuit which may be given primary consideration. The first is that the time constant of resistor R2 and capacitor C2 should be large enough not to turn on switch Q1 right away after the input power source is connected. The second one is that the gate voltage of switch Q1 should be higher than the turn-on threshold voltage of switch Q1 during normal operation so that switch Q1 does not carry any unintended loss. The third one is the time constant of resistor R3 and capacitor C2 should be as short as possible, so that switch Q1 can turn off when the input power is removed.
However, because switch Q1 is controlled passively, the discharging time of the gate voltage of switch Q1 when the input power source is disconnected depends primarily on the time constant of resistor R3 and capacitor C2, which cannot be very small due to the second of the design constraints mentioned above. That is, switch Q1 will stay on for a relatively long time after the input power source is disconnected. If the input power source is momentarily disconnected during normal operation due to for example blackout, etc., the ballast will continue to deliver energy to the HID lamp from capacitor C1. The voltage on capacitor C1 will reduce sharply. In this situation, because of the switch Q1 being on, there is no resistive impedance in series in the input circuit, and the inrush current may be very high when the input power source is reconnected. This high inrush current flows through the switching element Q1 to charge the tank circuit C1, which is a major concern not only for the inrush current itself, but also for the operational reliability of switch Q1.
FIG. 2 shows another example of an inrush protection circuit 21 configuration alongside a boost chopper as the PFC circuit in an electronic ballast 2. This inrush protection circuit 21 is more intelligent compared with the example 11 shown in FIG. 1, but also more expensive. The turn-on of switch Q1 is passively controlled by the input voltage, while the turn-off of switch Q1 is actively controlled by signal IC2-1(-2). In addition to the circuit arrangement of the previous example, the controller IC1 is needed to sense the voltage across C1 and forces switch Q1 to turn off when the voltage on capacitor C1 becomes low by shorting capacitor C2 with signal IC2-2. R4 is a resistor provided to regulate the current of IC2-1.
When the input power source is connected to the ballast 2, the gate voltage of switch Q1 is gradually charged up in substantially the same way as in the example of FIG. 1. When the input power source is disconnected, switch Q1 can be turned off immediately and no high peak inrush will occur whenever the input power source is re-connected.
However, as previously stated this circuit 21 is more expensive because of the added components controllers IC1 and IC2. The circuit arrangement is also more complex. Controller IC1 may be utilized for other functions of the ballast, such as the output power regulating controller, but the design layout may be difficult since controller IC1 now needs to be dedicated not only to the output side of the ballast but also to the input side. Controller IC1 is also required to have one extra pin to control controller IC2. An isolated device, controller IC2, such as an opto-coupler is needed because the circuit GND which connects to controller IC1 is the drain of switch Q1, not the source.
The conventionally known examples 11 and 21 as described herein are therefore either lacking in certain desired functionality or prohibitively expensive and complex.